i am both a PC game lover and a halo lover. Its just like what has been said before, with Halo you can go anyway, say to college, and find people with an xbox and meet in the dorm lounge and have fun all day.
I dont care what PC sackriders say, you cannot do this with a PC. Yes, you can lug around your PC with you, yes, you can buy a sweet laptop and do the same, but, not that is not as universal as consoles.
I love playing BF:2 online, and i would rather play more single player FPSs on PC. I loved HL:2, i enjoyed FEAR, etc. but there is something that is much more fun about playing halo in the same room as 15 other people, something that doesnt translate with VOIP and chat text.
There is nothing especially awesome or revolutionary about Halo, it is simply a well made game that is very fun to play.